NOT APPLICABLE
This invention relates generally to antennas for wireless communications, and more particularly the invention relates to a compact antenna structure which provides improved RF isolation between a receiver and a transmitter sharing the antenna.
High gain antennas are widely used for communication purposes and for radar or other sensing use. In general, high antenna gains are associated with high directivity, which in turn arises from a large radiating aperture. A common method for achieving a large radiating aperture is through the use of parabolic reflectors fed by a feed subarray located at the focal point of the parabolic reflector. Modern communication and sensing systems are finding increasing use for antenna arrays for high gain use. An antenna array includes an array of usually identical antennas or elements, each of which ordinarily has lower gain than the array antenna as a whole. A salient advantage of an array antenna is the ability to scan the beam or beams electronically without physically moving the mass of the reflector or array.
A circularly polarized antenna element used for circular polarization is described in Volman U.S. Pat. No. 6,172,655, issued Jan. 9, 2001. This patent describes an array antenna in which circular polarization is achieved by the use of ultra short axial mode helical antenna elements. In the Volman arrangement, the axial mode helical antenna element has a one-port design, that is the transmitter and receiver are connected to the antenna through the same port. The one-port helical antenna requires multiplex equipment for combination or separation of frequency bands in order to provide separation/combination of the receive and transmit bands. The receive/transmit band of multiplexers for space communication systems must provide extremely high isolation between the bands (on the order of 120 dB or higher) owing to the large difference of the receive and transmit signal levels. Further, the lowest possible insertion loss, mass, and size must be provided along with high power handling capability without multipactor breakdown. Additionally, pulse interval modulation (PIM) must be reduced to a level below 180 dB.
FIG. 1 is a functional block diagram of a conventional multifold diplexer for connecting a transmit signal from filter 10 to antenna 12 and connecting a received signal through receiver filter 12. The receiver and transmitter filters are tuned respectively to the receive frequency band and the transmit frequency band that are directly connected to the T-junction 16 of the antenna. However, in this arrangement almost full transmitter power goes straight to the receiver filter. In order to mitigate the PIM and multipactor effects, the receiver filter must handle the full transmitter power level, which presents a high risk element with extra mass, size, and weight in the antenna. Further, the coaxial T-junction used at UHF and other frequency bands is a high risk element because of possible PIM problems.
FIG. 2 is a functional block diagram of a conventional diplexer with directional filter modules. This arrangement requires two receive filters 20, 22, a first hybrid 24 for connecting the receive filters to a receiver, and a second hybrid 26 for connecting the receiver channel and transmit filter 28 in the transmit channel to the transmit/receive antenna. This arrangement has high manufacturing and tuning expense as well as high mass and large size. This arrangement does provide approximately 20 dB of increased isolation between the transmitter output and receiver input when compared to the manifold diplexer of FIG. 1. However, full transmitter power again goes to the receive filter input, thus creating the possibility of PIM and multipactor problems.
In accordance with the invention, a dual port antenna and array are provided for increased RF isolation between the receiver channel and the transmitter channel and between the receive and transmit frequency bands.
The antenna includes a helical antenna element and a dipole element inside of the helical element, without touching the helix, and with a separate port to each element. The helical element is supported by and extends from a ground plane, and the dipole is located about a quarter wavelength (receive band) above the ground plane. Advantageously, a balan/two wire line for the port to the dipole element can be printed on a printed circuit board of the ground plane support structure.
The two ports eliminate any galvanic contact between the transmitter and receiver circuits. Lower mass and a compact size are realized with the structure which leads to greater flexibility and packaging and thermal environment in a spacecraft application, for example.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.